Important to me:
1) Selection. Obviously, if they don't have it on a regular basis, I won't go back.
2) New stuff in fast. 3 game stores are in my city, and they're all competing for my gaming dollars. They're all generally the same, except for a few issues. This is one of them. Whoever gets the new stuff in first, wins.
3) Location. Of the 3 game stores, one is on my drive home from work. One is 5 minutes (non-rush-hour) from my office, but in the opposite direction. The last is on the other side of the city. I will generally avoid the one on the other side of the city, but will go there if they get the books in first (they also have a 10% discount, but that doesn't make much of a difference because of the store's location).
4) Staff Competence. They don't need to be terribly friendly, just "nice enough" and competent. Know what the heck is coming out for both D&D and d20 at the very least. They don't even need to know my name (however, the franchise manager of the EB nearest to me - in a major city mall, no less - somehow knows my name. I think that says a little too much of my buying habits...). :/